Oscillators are electrical devices that generate an oscillating or repetitive signal. The signal comprises a voltage which varies in magnitude and sign over time. The signal can be a sinusoidal wave, such as in an analog signal, or a square wave, such as in a digital electronic signal. Signals generated by an oscillator, especially electronic signals, have a number of applications such as, for example, a precise reference clock source in a voltage-controlled oscillator for frequency tuning, as a reference lock source in a phase-locked loop (PLL) for locking onto another signal, or a frequency synthesizer to generate many other frequency references required in specific applications including microprocessors, wireline (tethered) or wireless communication systems, and application-specific integrated circuits (ASICs).
Oscillators comprise a resonator and an oscillator core. The resonator creates the oscillations and the oscillator core provides power to the resonator to initiate and sustain oscillations. A resonator can be, for example, an inductor-capacitor (LC) resonator or an electro-mechanical resonator. LC resonators comprise an inductor and a fixed capacitor. A variable capacitor can also be added to an LC resonator to tune the frequency of oscillations produced by the LC resonator and oscillator. The use of an electro-mechanical resonator, such as a piezoelectric resonator, in place of an LC resonator can improve the quality (phase purity) of the oscillations in an oscillator. The quality factor of a resonator determines how under-damped its oscillator is—the higher the quality factor, the lower the rate of energy loss relative to the stored energy of the resonator. LC resonators in an integrated circuit (IC), for example, have a quality factor between 5 and 25. The quality factor of an electro-mechanical resonator can be 10 to 100 times higher than that of an integrated LC resonator.
When an electro-mechanical resonator is used with a differential oscillator, that has a common-source cross-coupled transistor oscillator core, to produce balanced oscillations, however, issues are introduced with respect to the oscillator latching to a static direct current (DC) state. Unlike an LC resonator, an electro-mechanical resonator has a very high impedance at low frequency and acts like an open circuit at DC. Although not an issue for single-ended oscillators, the high impedance at DC causes the cross-coupled transistors in a differential oscillator to become a latch with a very high DC gain so as to prevent the oscillations from starting in the oscillator. Accordingly, electro-mechanical resonators are commonly used in three-point oscillator topologies, such as Colpitts, Pierce, and Hartley oscillators, which do not suffer from the latching problem. A three-point oscillator, however, only provides a single-ended output signal, not a differential output signal. The differential output signals, as produced by a cross-coupled oscillator, have a better common-mode noise rejection and an increased oscillation swing across the resonator as compared to the single-ended output signal.
One known approach to address the latching issue is to place a capacitor at the source of cross-coupled NMOS transistors. This breaks the loop formed by the transistors and the resonator at DC, while closing the loop as desired at high frequencies. This approach, however, cannot be used with cross-coupled complementary oscillators comprising a pair of NMOS and PMOS transistors. There are advantages to using complementary cross-coupled inverters in an oscillator such as, for example, boosting transconductance (gm) and improving phase noise. Furthermore, the approach of placing a capacitor at the source requires more design effort to ensure stability because it does not unconditionally inhibit unwanted parasitic relaxation oscillations from occurring in the oscillator. Adding capacitors also causes some amount of decrease in signal swing and phase noise performance of the oscillator. Whether relaxation oscillations occur depends on the resistance and capacitance values in the DC blocking path in the oscillator. Stability analysis can be performed to determine the largest capacitor possible to avoid relaxation oscillations, but at the expense of lower signal swing and worse phase noise performance, as well as increased design complexity. Accordingly, it would be desirable to have a cross-coupled complementary oscillator comprising an electro-mechanical resonator that does not latch to DC or experience relaxation oscillations.
Furthermore, when an electro-mechanical resonator is used in a voltage-controlled oscillator, issues are introduced with respect to the tuning range. Specifically, the oscillator has a narrow tuning range due to the maximum-to-minimum capacitance ratio. Accordingly, it would also be desirable to have a voltage-controlled oscillator comprising an electro-mechanical resonator with an extended tuning range.